ÇÅÏÐÁÉÎÔ ÆÉÌÅ ÆÏÒÍÁÔ

     ÃARDS AND COLOR ON THE C64 (BACKGROUND).
ÔHE GRAPHICS MODE USED BY ÇÅÏÓ ON THE C64 SUPPORTS 320 BY
200 RESOLUTION, ORGANIZED AS A 40 BY 25 MATRIX OF 8 BY 8
PIXEL CARDS.  ÔHESE CARDS ARE STORED IN THE FRAME BUFFER
FROM LEFT TO RIGHT, TOP TO BOTTOM.  ÔHE DIAGRAM BELOW
BETTER ILLUSTRATES THIS FORMAT.

ÏÎ ÓÃÒÅÅÎ: 
CARD0   CARD1   CARD2  ...   CARD39 
CARD40  CARD41  CARD42 ...   CARD79 
 .
 .
CARD960 CARD961 CARD962 ...  CARD999

ÅÁÃÈ ÃÁÒÄ:  BYTE0
            BYTE1
            ...
            BYTE6 
            BYTE7 
     ÅACH BYTE CONTAINS 8 BITS, 1 PER PIXEL ON THE SCREEN.
ÔHESE EIGHT BYTES MAKE UP AN 8 BY 8 PIXEL IMAGE.

ÉÎ ÆÒÁÍÅ ÂÕÆÆÅÒ: 
               ÃÁÒÄ0    ÃÁÒÄ1    ÃÁÒÄ2
ÃÁÒÄ ÒÏ× 0 ->       BYTE0   BYTE8   BYTE 16 
          ...      ...      ... 
          BYTE7    BYTE15  BYTE23 

               ÃÁÒÄ40   ÃÁÒÄ41   ÃÁÒÄ42 
ÃÁÒÄ ÒÏ× 1 ->     BYTE320  BYTE328 BYTE336 
           ...      ...      ...
          BYTE327  BYTE335 BYTE443

     ÔHIS GRAPHICS MODE SUPPORTS 16 COLORS, BUT THE COLOR
RESOLUTION IS LIMITED TO CARD BOUNDARIES.  ÔHAT IS, TWO
COLORS CAN BE SPECIFIED FOR EACH CARD ON THE SCREEN -- A
BACKGROUND COLOR (BITS SET TO 0 IN THE CARD) AND A
FOREGROUND COLOR (BITS SET TO 1 IN THE CARD).  ÔHIS COLOR
INFORMATION IS STORED SEPARATELY FROM THE FRAME BUFFER,
ONE BYTE PER CARD.  ÔHE HIGH 4 BITS IN THE BYTE SPECIFY
ONE OF 16 COLORS TO BE USED FOR THE FOREGROUND, AND THE
LOW FOUR BITS SPECIFY THE BACKGROUND COLOR.
ÇÅÏÐÁÉÎÔ ÆÉÌÅ ÆÏÒÍÁÔ 
-------- ---- ------
     Á GEOÐAINT DATA FILE IS A ÖÌÉÒ FILE WITH 45 RECORDS.
(ÆOR INFO ON WHAT A ÖÌÉÒ FILE IS, SEE THE ÇÅÏÓ ÐROGRAMMER'S
ÒEFERENCE ÍANUAL).  ÅACH RECORD HOLDS THE IMAGE AND COLOR
DATA FOR AN AREA OF THE PAGE 16 LINES HIGH BY 640 PIXELS
WIDE.  Á GEOÐAINT DOCUMENT IS 720 LINES HIGH BY 640 PIXELS
WIDE, SO EACH RECORD REPRESENTS TWO CARD ROWS OF THE
ENTIRE DOCUMENT.  ÔHE IMAGE DATA IS STORED IN THE SAME
CARD FORMAT USED BY THE 320 BY 200 GRAPHICS MODE ON THE
C64, DISCUSSED ABOVE.
     ×HEN A GEOÐAINT FILE IS CREATED, ALL 45 RECORDS ARE
MARKED AS USED IN THE INDEX TABLE OF THE ÖÌÉÒ FILE.  ÔHE
TRACK AND SECTOR POINTERS ARE SET TO $00,$ÆÆ TO INDICATE
AN EMPTY RECORD; INSTEAD OF $00,$00 WHICH INDICATES AN
UNUSED RECORD.
     ÔHE 45 RECORDS ARE ASSIGNED TO ALTERNATE CARD ROWS,
FROM TOP TO BOTTOM ON THE DOCUMENT.  ÔHAT IS, RECORD 0
CONTAINS THE IMAGE AND COLOR DATA FOR CARD ROWS 0 AND 1;
RECORD 1 FOR CARD ROWS 2 AND 3, ETC.
     ÔHE IMAGE AND COLOR INFO IS COMPACTED BEFORE BEING
WRITTEN TO THE RECORD.  ÔHE PRE-COMPACTED LAYOUT OF THE
DATA IS:

#BYTES  START/END ($)  DESCRIPTION 
------  -------------  ----------- 
 640     000 - 27Æ     IMAGE DATA FOR 
                       1ST CARD ROW 

 640     280 - 4ÆÆ     IMAGE DATA FOR 
                       2ND CARD ROW 

   8     500 - 507     ALL ZEROES. 

  80     508 - 557     COLOR INFO FOR 
                       FIRST CARD ROW 

  80     558 - 5Á7     COLOR INFO FOR 
                       2ND CARD ROW

     ÔHE EIGHT BYTES OF ZERO WERE INCLUDED IN GEOÐAINT 1.0
FILES THROUGH AN OVERSIGHT, AND RETAINED AFTER THAT FOR
COMPATABILITY.
ÃÏÍÐÁÃÔÉÏÎ ÆÏÒÍÁÔ 
---------- ------
     ÔHE COMPACTION ROUTINE OPERATES ON THE ENTIRE 1448
BYTES AT ONCE, RESULTING IN A STRING OF BYTES IN THE
FOLLOWING FORMAT.

  ---------------------
  ! COMMAND/COUNT ! DATA !
  !     BYTE      ! BYTE !
  --------------------- ......

     ×HERE THERE IS ONE COMMAND/COUNT BYTE, AND FROM 1 TO
63 DATA BYTES.  ÔHIS PATTERN IS REPEATED UNTIL ALL 1448
BYTES ARE REPRESENTED.  ÔHE COMMAND/COUNT BYTE IS
INTERPRETED AS FOLLOWS:

HEX VALUE   INTERPRETATION 
---------   -------------- 
$00         ÅND OF RECORD 

$01 - $3Æ   ÕSE COMMAND BYTE AS ÃÏÕÎÔ.
            ÔHE FOLLOWING ÃÏÕÎÔ DATA 
            BYTES ARE UNCOMPACTED. 

$40         ÉNVALID COMMAND BYTE. 

$41 - $7Æ   ÓUBTRACT $40 FROM THE
            COMMAND BYTE, USE THE
            RESULT AS ÃÏÕÎÔ. ÔHE NEXT
            8 DATA BYTES COMPRISE A
            CARD TO REPEAT ÃÏÕÎÔ TIMES.
            ÔHESE 8 DATA BYTES EXPAND 
            TO 8*ÃÏÕÎÔ IMAGE BYTES. 

$80         ÉNVALID COMMAND BYTE. 

$81 - $ÆÆ   ÓUBTRACT $80 FROM THE
            COMMAND BYTE, USE THE
            RESULT AS ÃÏÕÎÔ. ÒEPEAT THE
            SINGLE DATA BYTE THAT
            FOLLOWS ÃÏÕÎÔ TIMES. ÔHIS
            SINGLE BYTE EXPANDS TO
            ÃÏÕÎÔ IMAGE BYTES.